The present invention relates to a focusing state determination device for a taking lens, and in particular to a focusing state determination device for a taking lens which is applicable to focus determination in autofocus control for a taking lens.
Japanese Patent Application Publication No. 55-76312 and Japanese Patent Publication No. 7-60211 disclose methods of determining focusing states (front focus, rear focus, and just focus) of a taking lens using a plurality of imaging elements having different optical path lengths. For example, two focusing state determination imaging elements for capturing images of an identical viewing area are arranged with respect to an imaging element for capturing images for video (video imaging element) at a position where the optical path length is longer than that of the video imaging element and at a position where the optical path length is shorter than that of the video imaging element, respectively. Then, high frequency components are extracted from image signals obtained by these focusing state determination imaging elements, focus evaluation values indicating degrees of focus (contrast of images) for image capturing surfaces of the focusing state determination imaging elements are found on the basis of the high frequency components, and the focus evaluation values are compared. Consequently, it is determined, from a relation of magnitude of the focus evaluation values, in which state of front focus, rear focus, and just focus a focusing state on the image capturing surface of the video imaging element is. By applying such a determination method of a focusing state, focus determination for autofocus can be performed, and not only whether or not a focusing state is in just focus but also whether the focusing state is front focus or rear focus can be determined. Thus, there is an advantage that a response speed to focus is high.
Japanese Patent Application Publication No. 59-128506 discloses an optical device which branches a part of light beams passing through a taking lens into linear light receiving device array with an oblong light beam division nirror, which is arranged at a position deviated sideways from the optical axis of the taking lens, and subjects outputs of devices of the linear light receiving device array to comparison operation to thereby determining a focusing state.
In recent years, performance of a taking lens and a camera main body used in a television camera system has been remarkably improved as the high-definition television broadcast is becoming popular, and a situation occurs in which a cameraman insufficiently performs focus adjustment with his/her naked eyes with reference to an image in a view finder only. Thus, necessity for autofocus, which is rarely used in a conventional television camera system, is increasing, and highly accurate focusing state determination for the autofocus is becoming indispensable.
In the case where an optical system for focusing state determination is formed in a camera main body, focusing state determination can only be performed by the camera main body. On the other hand, in the case where an optical system for focusing state determination is formed in the taking lens as in the optical device disclosed in Japanese Patent Application Publication No. 59-128506, there is an advantage that focusing state determination is possible in all camera systems using the taking lens.
However, in the optical device disclosed in Japanese Patent Application Publication No. 59-128506, since light in an entire wavelength range of subject light for video passing through the taking lens is split from the subject light to be used as subject light for focusing state determination, there is a disadvantage that sufficient accuracy cannot be obtained for focusing state determination applicable to the high-definition television broadcast or the like.
The present invention has been devised in view of such circumstances, and it is an object of the present invention to provide a focusing state determination device for a taking lens, which incorporates an optical system for focusing state determination in the taking lens and makes it possible to perform sufficiently highly accurate focusing state determination applicable for the high-definition television broadcast or the like.
In order to attain the above-described object, a focusing state determination device for a taking lens according to the present invention is characterized by comprising: a plurality of focusing state determination imaging elements which are arranged at positions having optical path lengths with respect to the taking lens different from each other; and a light splitting device which is arranged on an optical path in the taking lens, and splits light of color components in a wavelength range of 500 mm to 600 nm in subject light passing through the taking lens into subject light for video entering an imaging element for video of a camera main body and subject light for focusing state determination incident on the plurality of focusing state determination imaging elements, wherein a focusing state of the taking lens is determined according to high frequency components of image signals obtained by the plurality of focusing state determination imaging elements.
Preferably, the light splitting device is arranged in a relay optical system of the taking lens.
Preferably, the focusing state determination device for the taking lens is applied to focus determination in autofocus control of the taking lens.
According to the present invention, light of color components in the wavelength range of 500 nm to 600 nm, that is, green light is used as subject light for focusing state determination. In general, a luminance signal in a transmitted image indicates brightness and darkness of an image and is generated with mixture of signals of the primary colors such that the green components exist in the largest quantity while taling into account the fact that sensitivity of human eyes for brightness is the highest for green among the three primary colors of red, green and blue. Therefore, by using green light as subject light for focusing state determination as in the present invention, highly accurate focusing state determination more conformable to sensitivity of the eyes becomes possible, and highly accurate focus can be obtained by applying the focusing state determination of the present invention to focus determination in autofocus and the like.
Moreover, by arranging the light splitting device for splitting subject light for focusing state determination from subject light for video in the taking lens, an optical system for the focusing state determination device in accordance with the present invention can be incorporated in the taking lens separately from a camera main body, and focusing state determination becomes possible in all camera systems using the taking lens.